PURPOSE: To measure the gap between measuring surfaces accurately and quickly by separately detecting the distances from a reference surface to two opposing measuring surfaces by a detector, and computing and processing the outputs.

CONSTITUTION: A rotary shaft 14 of rotary transformers 1 and 2 of an object to be measured is coupled with the circular cylinder 13 of a holding device 9 and held by it. At this time the reference surface 36 of a bush 15 is intimately contacted with a contact surface 3a. A detecting piece 32 is contacted with the measuring surface 31 of the rotary transformer 1. A pulse motor 17 is driven and rotated by a control signal from a CPU. An arm 24 is turned from the vertical direction to the horizontal direction through a coupling 18 and worm 29. The transformer 1 is pushed by the arm 24, and the reference surface 36 of the bush 15 is perfectly and intimately contacted with the contact surface 3a. At this time, a lower limit stopping piece 27 is engaged with a photoswitch 29 and a detected signal is outputted. When the pulse motor 17 is rotated in the reverse direction, a signal is outputted from an upper limit stopping piece 28. Both signals and a signal from from a detector main body 33 are computed and processed in the CPU, and the gap (g) between the rotary transformers 1 and 2 is measured quickly and accurately.